Inkjet printers are commonplace in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684). Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes a printing medium, such as paper.
An inkjet printer produces a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more print cartridges each having a printhead with ink ejecting nozzles. The carriage traverses over the surface of the print medium. An ink supply, such as an ink reservoir, supplies ink to the nozzles. The nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller. The timing of the application of the ink drops typically corresponds to the pattern of pixels of the image being printed.
In general, the small drops of ink are ejected from the nozzles through orifices or nozzles by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as small thin film resistors. The small thin film resistors are usually located adjacent the vaporization chambers. Heating the ink causes the ink to vaporize and be ejected from the orifices (also known as "firing").
Specifically, for one dot of ink, a remote printhead controller, which is usually located as part of the processing electronics of the printer, activates an electrical current from an external power supply. The electrical current is passed through a selected thin film resistor of a selected vaporization chamber. The resistor is then heated for superheating a thin layer of ink located within the selected vaporization chamber, causing explosive vaporization, and, consequently, a droplet of ink is ejected through an associated orifice of the printhead.
One problem common in inkjet printers that reduces the efficiency of an ink drop ejection, however, is scan axis directionality (SAD). The scan axis is the axis along which the printhead and carriage move during various operations such as a printing operation. SAD is the measure of error (in degrees) in an ejected ink drop trajectory with respect to the scan axis. In general, SAD occurs when an ejected ink drop does not land on the print media (such as paper) in the desired location. The trajectory error reduces the accuracy and efficiency of the printing operation.
Another problem common in inkjet printers is radiation interference. In general, a great deal of power is required over a short period of time in order to vaporize each ink drop. The switching of power on and off over a short amount of time generates undesirable electromagnetic radiation interference (EMI). When this switching is performed simultaneously on a large number of vaporization chambers, the EMI problem is intensified. In effect, EMI causes the wiring and cabling, delivering power from the power supply to the vaporization chamber, to radiate energy like an antenna. This radiation interferes with internal components of the printer and also electronic devices external to the printer.
A common method of reducing EMI is by shielding the printer cabling and wiring. However, shielding is expensive and adds weight and bulk to the printer. Shielded cabling and wiring is also stiff, unwieldy and constrains the movement of dynamic components inside the printer.
Therefore, what is needed is a new printing system and method including a printhead assembly that can efficiently control and optimize the firing operations of the printhead.